Delay setting the handler in Logger so each Logger can have its own handler

[lux]

Each Logger instance stores its handler and switches the handler used by Analog before logging, which I think might solve #49 with minimal changes.

Example usage:

<?php

require 'vendor/autoload.php';

use Analog\Logger;
use Analog\Handler;

$log1 = '';
$log2 = '';

$logger1 = new Logger ();
$logger1->handler (Handler\Variable::init ($log1));

$logger2 = new Logger ();
$logger2->handler (Handler\Variable::init ($log2));

$logger1->debug ('Log 1.1');
$logger2->debug ('Log 2.1');

$logger1->debug ('Log 1.2');
$logger2->debug ('Log 2.2');

$logger1->debug ('Log 1.3');
$logger2->debug ('Log 2.3');

echo $log1;
echo '---' . PHP_EOL;
echo $log2;